In systems analysis, a many-to-many relationship refers to a relationship between two entities (see also Entity-Relationship Model) A and B in which A may contain a parent row for which there are many children in B and vice versa. Because most DBMSs only support one-to-many relationships, it is necessary to implement such relationships physically via a third junction table, say, AB with two one-to-many relationships A -> AB and B -> AB. In this case the logical primary key for AB is formed from the two foreign keys (i.e. copies of the primary keys of A and B).
In web application frameworks such as CakePHP and Ruby on Rails, a many-to-many relationship between database tables in a model is sometimes referred to as a HasAndBelongsToMany (HABTM) relationship.
See also
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